At 05.23 hours on 4 March 1940 the unescorted Thurston (Master William Carr Fortune) was hit in the stern by one torpedo from U-29 and sank within one minute 32 miles west by north of Trevose Head. The master and 33 crew members were lost. Three crew members were picked up by the British steam merchant Moyle and landed at Cardiff.

Shortly after midnight on 4 March, the French steam merchant S.N.A. 1 (2679 grt), en route from Ardrossan to Rouen, collided with Thurston about 60 miles south of Milford Haven and sank in 50°33N/05°47W. 31 crew members were rescued by Thurston, but only two men survived the second sinking by rescuing themselves into an overturned lifeboat. They rightened it, but one of the men died of exposure before a fishing trawler found the last survivor after about 11 hours and brought him to a hospital where he had to stay for a week.